Known superchargers, or turbochargers, belong more or less exclusively to one of the following three basic types of superchargers. Superchargers presenting a turbine housing having one single spiral chamber, a turbine housing having two parallel spiral chambers, or a turbine housing having variable geometry.
In a turbine housing having one single spiral chamber the spiral chamber presents an inlet having a small area or an inlet having a large area in relation to the radius of the inlet of the spiral chamber, usually defined as small and large A/R-ratio, respectively. Turbine housing having a spiral chamber presenting a small inlet area in relation to the radius of the inlet of the spiral chamber are arranged to cause overcharge already at small exhaust gas flows, which as a rule coincide with low engine speed, but at big exhaust gas flows, which as a rule coincide with high engine speed, a large back pressure will be created in this type of turbine housing whereupon a large amount of the exhaust gas flow will be released via a waste gate and thereby a desired overcharge will not be attained at high engine speed. Turbine housing having one spiral chamber presenting a large inlet area in relation to the radius of the inlet of the spiral chamber are on the other hand arranged to create overcharge at high engine speed and large exhaust gas flow. However, this type of turbine housing presents worse efficiency or large overcharge delay which implies that none overcharge takes place at low engine speed and small exhaust gas flow.
In a turbine housing having parallel spiral chambers the turbine housing comprises two spiral chambers, which are separated by a radially extending wall and which have the same or different inlet areas and/or radius at the inlet thereof. However, it shall be pointed out that the common inlet area of the two spiral chambers in relation to the radius of the inlet of the spiral chambers usually is classified as large by comparison with a turbine housing having one single spiral chamber, thus the turbine housing of this type is suitable for large exhaust gas flow and high engine speed. Furthermore, both the first spiral chamber and the second spiral chamber mouth in parallel with each other around the entire turbine seat of the turbine housing. The function of the radially extending wall is to guide the exhaust gas flow coming from a first set of complementary cylinders to one spiral chamber and the exhaust gas flow coming from a second set of complementary cylinders to the other spiral chamber. By having only complementary cylinders, i.e. cylinders not having their exhaust gas outlet valves open at the same time, operatively connected to each other no back flow takes place from a first cylinder to a second cylinder having partly overlapping cycle with said first cylinder. In theory the turbine is thus alternately affected by the exhaust gas flow from one and the other spiral chamber, which gives a better utilization of the exhaust gas pulse energy even at low engine speed. Thus, a turbine housing having parallel spiral chambers provide a larger engine speed range within which a desired overcharge takes place, in relation to a turbine housing having one single spiral chamber.
In a turbine housing having variable geometry a great number of vanes are arranged at the interface between the spiral chamber and the turbine seat. The vanes are maneuverably movable between a more or less tangential position in relation to the turbine seat and a more or less radial position in relation to the turbine seat. At low engine speed and small exhaust gas flow the vanes are moved to the tangential position and at high engine speed and large exhaust gas flow the vanes are moved to the radial position. Thanks to the variable geometry a desired overcharge takes place already at very low engine speed at the same time as a desired overcharge also takes place at high engine speed. However, this solution is very expensive and construction wise complicated due to the great number of small movable parts in combination with high operational temperatures existing in turbine housing during operation. This applies in special to Otto-engines generally having higher exhaust gas temperatures than diesel engines. Thus, a turbine housing having variable geometry is uncommon in connection with Otto-engines.